In the past, they have dealt with this by setting a date for the game key to be added to the Bnet account. If added by X date it counts for the pre-purchase bonus.
Example cited is D3 Reaper of Souls expansion (2014).
There were several bonuses and some cross game rewards. Here is the key text of the arrangement.
If you’ve already digitally pre-purchased the Reaper of Souls expansion, either as the standard edition or the Digital Deluxe Edition, then you’re all set! These bonus items will be granted to your Battle.net account as follows:
- The Wings of Valor will be automatically sent to your existing and any newly created Diablo III characters via an in-game notification starting March 20, 2014.
- Valla the Demon Hunter will be available once Heroes of the Storm has been officially released. Note that Heroes of the Storm is still deep in development and does not have an announced release date, but you can stay tuned to the official game site for future updates. Note also that pre-purchasing Reaper of Souls does not guarantee a spot in the eventual Heroes of the Storm alpha or beta tests.
If you pre-order Reaper of Souls at a retail store—either a physical standard edition or physical Collector’s Edition—be sure to redeem your Reaper of Souls game key via your Battle.net account on or before 11:59 PM PST on March 31, 2014. If you redeem your game key after that date, your pre-order loot will be consumed by the fires of the Burning Hells and lost forever. While you have six days from the time the expansion launches on March 25 to the time you must enter your code, to be safe, you should secure your preorder today and be ready to input your game key on launch day.
I don’t know if Blizzard will do the same for Dragonflight - we need to wait and get clarification, however there is a past model they can use.